1. Field of the Invention
The present invention relates generally to the field of optical imaging, and more specifically to catadioptric optical systems used for bright-field and dark-field optical inspection applications.
2. Description of the Related Art
Many optical and electronic systems inspect surface features for defects such as those on a partially fabricated integrated circuit or a reticle. Defects may take the form of particles randomly localized on the surface, scratches, process variations such as under etching, and so forth. Such inspection techniques and devices are well known in the art and are embodied in various commercial products such as many of those available from KLA-Tencor Corporation of San Jose, Calif.
Several different imaging modes exist for optical inspection. These include bright-field and various dark-field imaging modes. Each of these imaging modes is provided to attempt to detect different types of defects. The oblique dark-field mode is one of the most sensitive and stable dark-field modes. The requirements for a state of the art semiconductor inspection system include high numerical aperture (NA), large field size, reasonable bandwidth, and UV-DUV wavelength. When combined with the beam delivery and low scattering requirements of oblique dark-field imaging, finding a suitable design for advanced specimen inspection becomes very challenging.
Examples of optical systems that can support semiconductor inspection include high NA UV-DUV catadioptric systems with large field sizes are shown in U.S. Pat. No. 5,717,518 by Shafer et al., and U.S. Pat. No. 6,064,517 by Chuang et al.
These previous designs can, in certain circumstances, exhibit beam delivery issues when performing oblique dark-field imaging. In these systems, one method for implementing the oblique dark-field mode uses a collimated beam of monochromatic light illuminating the wafer from inside the optical system within the NA defined by the objective. However, small amounts of scattered and reflected light from lens elements using this design can produce optical noise at levels that compromise sensitivity. Laser illumination can be introduced near the pupil in the focusing lens group or from an alternate pupil location within the catadioptric group. These methods of illumination can cause a significant amount of back-scattered and reflected light from the multiple lens surfaces traversed by the illuminating light. Forward-scattered light from the specularly reflected component off the wafer can also be a significant potential problem.
One previous method for achieving oblique laser dark-field illumination and imaging uses a collimated beam of monochromatic light to illuminate a semiconductor wafer from or originating outside the imaging objective. This mandates use of a long working distance objective to allow access by the laser to the area of interest on the semiconductor wafer. Objectives used in dark-field applications of this type are generally refractive objectives limited to NAs less than 0.7, corresponding to collection angles of only up to 44 degrees from normal. A major drawback of this approach is the small imaging NA that limits the amount of scattered light that can be collected. Another drawback is the small spectral bandwidth and small field size that are typical of refractive UV-DUV objectives.
Another previous solution uses a catadioptric objective with a nosepiece to allow external oblique illumination. Such objectives exclusively employ spherical optical surfaces and have certain issues when implemented.
It would therefore be desirable to have a system for inspecting a specimen that improves upon the systems previously available, and in particular for enabling inspection of specimens such as wafers using an oblique dark-field inspection mode. It would be particularly desirable to offer systems or designs that may be used under various circumstances and with various components that overcome the imaging issues associated with previously known designs.